The successful application of semiconductor technology to the fabrication of commercial electrical devices, and the recent strides in developing ultraminiature "chips", have been in part due to advanced semiconductor purification and doping processes. When ultra-high purity is required, the semiconductor materials may be purified by means of a zone refining technique originally disclosed by W. G. Pfann, in U.S. Pat. No. 2,739,088 issued Mar. 20, 1956. The zone refining technique makes use of the solute-solvent phase relationships characteristic of the semiconductor material. The technique depends on the fact that the semiconductor material can sustain a higher solute level in the molten state than it can in the solid state. Consequently, passing a molten zone through the semiconductor material may result in a wake or resolidified material which possesses a lower impurity concentration than the unprocessed semiconductor material. Impurities present in the material prior to processing are "caught" in the traversing molten zone and, because of the phase properties of the material, may appear in lower concentrations in the resolidified material subsequent to passage of the molten zone.
In the zone refining process, motion of the molten zone through the starting material is obtained by relative motion between the material and an appropriate heat source which produces the molten zone. In a subsequent disclosure contained in U.S. Pat. No. 2,813,048, W. G. Pfann discussed a method of zone melting in which relative motion between the heat source and the substrate is not necessary. In this new "temperature gradient zone melting process" a temperature gradient is established across the substrate resulting in the establishment of a molten zone, and its traversal across the substrate from lower temperature to higher temperature regions. The molten zone migrates under the influence of a driving force associated with diffusion within the molten zone, which arises from a concentration gradient which exists across the molten zone. The temperature gradient zone melting process, like the previously disclosed zone refining process, can be used both for purification of impure substrates, as well as for doping pure substrates. The temperature gradient zone melting process is now well established art in the field and the various physical and chemical phenomena associated with it are essentially fully understood (see, for example, W. G. Pfann, Zone Melting J. Wiley & Sons, 1958; L. H. Van Vlack, Elements of Materials Science, Addison Wesley, 1959 p. 185ff).
The temperature gradient zone melting process has been further developed by many workers in the field. These developments are discussed, for example, in a series of disclosures assigned to the General Electric Company (e.g., U.S. Pat. No. 3,898,106 and references cited therein). These embodiments of the temperature gradient zone melting process have been referred to as thermomigration--referring to the migration of the molten zone under the influence of an appropriate temperature gradient. These processes still require the establishment of a temperature gradient across the substrate as disclosed originally by W. G. Pfann in U.S. Pat. No. 2,813,048. However, in these later developments, elaborate steps are taken to avoid instabilities and non-uniformities in the process, and to increase the migration speed of the molten zone through the substrate.
Recently, the laser has been applied to semiconductor processing in a number of different areas. For example, the laser has been used to heat processed semiconductor materials so as to anneal imperfections in the crystal structure which arise during required processing steps. Such laser annealing processes are discussed for example, in U.S. Pat. No. 4,154,625 (Golovchenko-Venkatesan).
A somewhat related process is discussed in U.S. Pat. No. 3,940,289 which involves a flash melting method for producing impurity distributions in solids. In that process, at least a portion of the substrate is melted under the simultaneous influence of a heat sink and laser irradiation. Upon refreezing, the original dopant concentration of the substrate is altered, in part because of the diffusion properties of the dopant in the molten state, and in part because of the effect of the refreezing interface.